Physiologically Controlled Cardiac Pacer

Abstract

A cardiac pacer is described which uses a physiological function such as breathing rate, to vary the production of electronic pulses which are fed to a constant current source connected to the ventricle. A constant source compensates for the fibrotic growths that often occur around the electrodes implanted in the heart. In another variation, the pulses are fed to two separate constant current sources, one connected to the atrium and the other, with delay, to the ventricle.

Description

BACKGROUND OF INVENTION

One may enumerate cardiac pacer classes in many ways depending upon the context of the point the author is trying to make. For this reason we choose to categorize pacers into the following classes: p-wave synchronous pacers, RF pacers and inductive-coupled pacers.

P-wave synchronous pacers function by detecting and amplifying the electrocardiographic p-wave and stimulating the ventricles via a delay mechanism. In this manner, heart rate is accommodated to the variations of the SA node.

Radio frequency cardiac stimulation provides a method free of the discomforts and dangers of long term, externally applied cardiac stimulation. This technique consists of transmitting electrical energy to the heart through radio waves via a two megacycle frequency generator and a transmitter antenna coil to a small receiving coil. Heart rate is externally controlled by the patient.

The inductive coupled pacer consists of a pulse generator, attached by a flexible lead to an external primary coil that is strapped to the skin over the implanted secondary coil, the ends of which are the myocardial electrodes. The pulse in the primary coil produces a pulse in the secondary coil by simple electromagnetic induction. Heart rate is externally controlled by the patient.

Artificial pacers are not self-regulatory. They are therapeutically applied to patients with disturbances of the conducting system. The origin of the conduction block may be from congenital heart disease, congenital AV block, surgical, or acquired nonsurgical block. In the latter case, the conduction system may thus be interrupted in myocarditis, endocarditis, coronary ischemia, tumor, sclerosis of the cardiac skeleton, fatty infiltration, uremia, and hemochromatosis. After the application of the artificial pacer, drugs are given to cause complete AV block. Under the influence of such drugs atrial fibrillation may occur. This would be disastrous for a p-wave synchrony pacer (an amplifier failure would give the same result).

A second limitation of present cardiac pacers arises from their inability to insure atrioventricular synchrony. It is a consequence of the artificial pacer that the need for atrioventricular synchronization arises. The procedure of applying the pacer does not include the excision of the sinoauricular node, thereby creating two cardiac pacers: the auricular, i.e., the SA node; and the ventricular, i.e., the artificial pacer. There is no guarantee that these pacers will beat in proper synchronization with one another.

A third limitation is concerned with automatically adjusting the intensity of stimulation to compensate for growths of fibrotic tissue around the myocardial electrodes.

SUMMARY OF INVENTION

We have found that if the pacer frequency is controlled by one of several interrelated physiological functions, the heart rate will be accommodated to the needs of the organism. Such interrelated functions can be described as follows:

a. Vasoconstriction, tachycardia, increase in respiratory minute volume and increase in skeletal muscle tone.

b. Vasodilation, bradycardia, decrease in respiratory minute volume and decrease in skeletal muscle tone.

In either group, when one reaction is stimulated, it is followed by the occurrence of the other three reactions. For example, when an individual changes from the lying to the standing position, the venous return is reduced and a drop in arterial pressure occurs. The organism compensates by vasoconstriction (in both, arteries and veins) and by an increase of the heart rate. This reaction is always accompanied by an increase in respiratory minute volume and an increase in skeletal muscle tone. The opposite happens when an individual changes from the standing to the lying position. The resulting increase in blood pressure is compensated for by vasodilation, and simultaneously by bradycardia, decrease in respiratory minute volume and decrease in skeletal muscle tone. Similar reactions are observed during muscular exercise: Vasoconstriction, tachycardia, increase in muscular tone.

We have found it preferable to use respiration as the controlling factor and for this purpose employ an impedance pneumograph. The waveform is converted into DC voltage which in turn fires a DC/pulse rate oscillator. The pulses are fed to either one constant current source which is connected to an electrode implanted in the ventricle of the heart or to two constant current sources, one connected to an electrode implanted in the atrium of the heart and the other delay-connected to the ventricle so that the atrium pulse is always ahead of the ventricle pulse.

The novel features of this invention include the enslavement of the cardiac frequency to a different physiological parameter, the stimulation of both the atrium and the ventricle, the use with such dual stimulation of a delay which varies in accordance with atrial frequency, the use of a constant current source for stimulating the heart, and the use of the combination of an impedance/voltage converter, frequency/DC converter and DC/pulse rate oscillator.

This invention is applicable to a pacer for a so-called artificial heart as well as a natural heart. The term "cardiac pacer" as used in the appended claims is intended to include an artificial heart.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic block diagram illustrating the invention.

FIG. 2 is a graph of normal heart rate as a function of respiration rate.

FIG. 3 is a graph of heart rate as a function of atrioventricular delay.

FIG. 4 is a circuit diagram of one example of this invention.

FIG. 5 is a circuit diagram of a constant current source.

SPECIFIC EXAMPLE OF INVENTION

As illustrated in FIGS. 1 and 4, electrodes are placed on the chest and connected to an impedance/voltage converter 11. Examples of impedance/voltage converters are described in the book by J. J. Nybor, "Electrical Impedance Plethysmography," the article, "Quantitative Evaluation of the Impedance Spirometry in Man," by Baker, Geddes, and Hoff in "American Journal of Medical Electronics," April--June 1965, pp. 73--77, and the article, "The Measurement of Physiological Events by Impedance Change," by Geddes and Hoff in "Proceedings of the San Diego Symposium for Biomedical Engineering," 1963, pp. 115--122. The output wave is then fed to a frequency/DC converter 12 to yield a DC voltage response to frequency. An example of a frequency/DC converter is described on pages 660--662 of the book, "Electronic and Radio Engineering," by F. E. Terman, 4th edition, McGraw-Hill, 1955. This DC voltage is then fed to a DC/pulse rate oscillator 13. An example of a DC/pulse rate oscillator is described in the article, "A Two-Transistor Analog to Frequency Converter," by J. L. Krasner, "Electronic Design," Nov., 1964, and also in block 13 of the circuit illustrated in FIG. 4 in the drawings. The frequency of pulses produced by oscillator 13 is dependent on the input DC voltage. The pulses are used in two ways with a constant current source to send impulses to the heart.

In one method illustrated by the dotted line in FIG. 1, the pulses are fed to a constant current source 17 to send impulses to an electrode implanted in the ventricle of the heart. An example of such a constant current source is illustrated in FIG. 5. In the other arrangement, delineated by the solid lines of FIG. 1, the pulses are fed to two branches, one being a constant current source 14 whose impulses are sent to an electrode imbedded in the atrium of the heart. The other branch consists of a variable delay 15 before permitting the pulses to feed the constant current source 16 whose output is connected to an electrode imbedded in the ventricle of the heart. This latter arrangement provides atrioventricular synchronization. Pulse shape is controlled by block 18 as shown in FIG. 4. As shown in FIG. 3, the required delay for proper synchronization varies according to the heart rate. The atrial rate may exceed but never be less than the ventricular rate. However, the atria, when not fibrillating, must be in phase with the ventricles according to the graph of atrioventricular delay versus heart rate shown in FIG. 3.

Synchrony is obtained by designing the artificial pacer to set a pace slightly in excess of that pace the normal SA node would set. In this manner, the pacer artificially takes over the regulation of the atrium, i.e. increases the slope of the curve shown in FIG. 2. The ventricles are stimulated by a variable delay mechanism (consistent with FIG. 3) which operates by changing the temporal relation between two pulse generators, regulated by the same physiological parameters, as a function of their frequencies. (See block 15 in FIG. 4) In the case of atrial fibrillation, artificial atrial control is biologically overridden. Also, the atrial rate can never be less than the ventricular rate.

We have determined that there is no apparent difference between normal heart rate and pulse frequency response when the subject lies, sits, or stands, or does light muscular exercise. There is, however, significant difference between heart rate and pacer frequency response during hyperpnoea, and heavy muscular exercise, as well as during application of pressure on the carotid sinuses (carotid sinus reflex). Accordingly, the pacer is so designed that its frequency output can not be slowed below a selected output, as for example 60 pulses per minute, or increased above a selected maximum, as for example 160 pulses per minute. For this purpose a relaxation oscillator can be used whose frequency increases as a linear function of voltage. Thus, if only positive or ground signals are applied, the pacer will not fire at less than the relaxation frequency. The result is that extreme bradycardia is avoided.

The term "pacer" is used in the title, abstract, specification and claims as the full equivalent of the generic term "pacemaker" to indicate a device for stimulating the heart with an alternating current to steady the heart or to reestablish the rhythm of an arrested heart, as defined in "Websters Third New International Dictionary" (Unabridged Copyright 1966). The term "Pacemaker" is a registered trademark for one form of such device.

Claims

We claim:

1. The combination of a first means responsive to a separate but normally heart-related physiological function and a cardiac pacer, said means controlling the output frequency of said pacer and comprising in series an impedance/voltage converter, a frequency/DC converter, and a DC/pulse rate oscillator.

2. Claim 1 wherein said function is respiration.

3. Claim 1 wherein a constant current source connects said oscillator to said pacer.

4. Claim 1 wherein said pacer includes a first constant current source whose output is adapted for connection to the atrium, a second constant current source whose output is adapted for connection to the ventricle, and a second means for delaying the output of the second source with respect to the output of said first source.

5. Claim 4, wherein said second means provides a delay which is a function of artificially stimulated atrial frequency.

6. Claim 1 wherein said pacer comprises a first output adapted for connection to the atrium, a second output adapted for connection to the ventricle, and a second means for delaying the second output with respect to the first output.

7. Claim 6 wherein said second means provides a delay which is a function of artificially stimulated atrial frequency.

8. Claim 1 wherein said combination comprises an impedance/voltage converter, the output of which is fed to a frequency/DC converter, whose output voltage is fed to a DC/pulse rate oscillator, which branches into two constant current sources; the output of one said source being adapted for connection to the atrium, the output of the second said source being adapted for connection to the ventricle; second means being provided for delaying the output of said second source with respect to the output of said first source.

9. Claim 8 wherein said second means provides a delay which is a function of artificially stimulated atrial frequency.

10. A cardiac pacer which includes two constant current sources, one whose output is adapted for connection to the atrium and a second whose output is adapted for connection to the ventricle; means being provided for delaying the output of said second source with respect to the output of said first source.

11. Claim 10, wherein said means provides a delay which is a function of artificially stimulated atrial frequency.